1. Field of the Invention
The present invention generally relates to apparatus for measuring the earth's magnetic field and, more particularly, is concerned with a triaxial magnetic heading sensing apparatus which provides for simultaneous nulling of orthogonally positioned magnetic field sensors.
2. Description if the Prior Art
Because of the importance of magnetic compass technology to vehicle navigation, there have been a large number of technological approaches taken to the design of electronic compass sensors. Some early approaches involved the use of electrical pick-off mechanisms to detect the position of a mechanical compass needle. Other mechanisms have involved the use of spinning search coils from which magnetic field direction can be derived from the phase of the induced voltage. More modern approaches have been primarily focused on electronic technologies that do not employ moving parts.
Among the electronic technologies that have been applied to low field magnetometry, and in particular to compass applications, the flux gate type magnetometer has received the greatest commercial success. Flux gate magnetometers employ high permeability magnetic cores that are stimulated by an applied AC field. In flux gate operation, the magnetic core material is made to saturate in alternating directions along the axis of the magnetic core in response to the applied field from the primary winding. A system of one or more secondary windings is used to detect the core saturation. In the presence of an external magnetic field the saturation characteristics are asymmetric and the degree of asymmetry provides a direct indication of the strength of the external magnetic field along the axis of the core. Commonly, two orthogonal flux gate magnetometers are used in a gimbal structure or three orthogonal magnetometers are used for strap-down applications. Two axes flux gate sensors have also been employed which utilize torodial cores and multiple secondary windings.
Flux gate magnetometer sensors are inherently sensitive to the magnetic properties of the core material. Exotic, low coercivity alloys are used for the flux gate core which must be precisely annealed to preserve the required characteristics. Seemingly innocuous chemical contamination of the material or mechanical manipulation can alter the core's magnetic properties significantly. Moderately complex circuitry is required to stimulate the core and interpret the second harmonic secondary signals. To obtain an approximate zero balance, the coil windings must be tediously adjusted to desired nulling points.
In light of these considerations it is relatively expensive to build a flux gate magnetometer based compass sensor which is accurate to the degree required for aircraft navigation over the typical temperature range for flight vehicles.